1. Field of the Invention
The present invention relates to a method for preparing a surface modification coating of metal bipolar plates. More particularly, the method is carried out by a combination of surface-activation techniques and low-temperature pack cementation to prepare metal bipolar plates suitable for fuel cells or appropriately applied as end plates for fuel cell stacks.
2. Description of Related Art
In recent years, substrates made of graphite have been generally used as bipolar plates in proton exchange membrane fuel cells (PEMFC). These graphite plates on which parallel or various shaped flow-field patterns are made by a milling machine, possess high corrosion resistance and good electrical conductivity. However, they have drawbacks, such as being difficult to be processed, brittle, expensive, bulky, and heavy, and thus can not be applied in mass production.
Therefore, cheap metal substrates are suggested to substitute expensive graphite bipolar plates so that costs can be dramatically lowered. Nevertheless, metal bipolar plates are exposed in a corrosive environment during operation of PEMFC, and a few cations are dissolved into produced water. The corrosion phenomena will result in deteriorated performance of fuel cells.
In general, surface modification coatings of metal bipolar plates can be prepared by physical vapor deposition (PVD), chemical vapor deposition (CVD), or thermo reactive deposition (such as pack cementation). Among them, the physical vapor deposition has shortcomings of, for example, requiring expensive equipment and complex steps. Besides, formed coatings have low adhesion to the plates under restrictions of deposition temperature. In the chemical vapor deposition, desired compounds can be deposited on a substrate heated to promote a reaction between the substrate and chemical reactants. Hence, the depositing temperature and the manufacturing cost of this process are relatively high, and the high-temperature process is easy to incur the deformation of substrate. Accordingly, both PVD and CVD processes do not satisfy the demand of economic mass production.
Traditional pack cementation is performed at a high temperature of 800-1200° C., and elements Cr, V, Ti, etc. permeate into substrates to form coatings containing compounds such as carbides, nitrides, and borides. The pack cementation is particularly suitable for carbon-containing steel substrates, and it is advantageous in low costs of equipment and preparing uniform surface modification coatings on complex-shaped workpieces. Notwithstanding, this process does satisfy the industry demand of economic mass production because the high-temperature operation of traditional pack cementation would cause phase transformation, deformation, and deterioration of steel substrates.
As a result, there is an urgent need to provide a new technique cooperating with pack cementation so as to solve the problems that the thickness of metal bipolar plates has gradually decreased and graphite bipolar plates are not suitable in the future.